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EP0133926B1 - Bottom electrode for a direct current arc furnace - Google Patents

Bottom electrode for a direct current arc furnace Download PDF

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Publication number
EP0133926B1
EP0133926B1 EP84108020A EP84108020A EP0133926B1 EP 0133926 B1 EP0133926 B1 EP 0133926B1 EP 84108020 A EP84108020 A EP 84108020A EP 84108020 A EP84108020 A EP 84108020A EP 0133926 B1 EP0133926 B1 EP 0133926B1
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EP
European Patent Office
Prior art keywords
bottom electrode
furnace
electrode
section
vessel
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
EP84108020A
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German (de)
French (fr)
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EP0133926A1 (en
Inventor
Karl Bühler
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BBC Brown Boveri AG Switzerland
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BBC Brown Boveri AG Switzerland
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Publication date
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Priority to AT84108020T priority Critical patent/ATE27059T1/en
Publication of EP0133926A1 publication Critical patent/EP0133926A1/en
Application granted granted Critical
Publication of EP0133926B1 publication Critical patent/EP0133926B1/en
Expired legal-status Critical Current

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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F27—FURNACES; KILNS; OVENS; RETORTS
    • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
    • F27D11/00—Arrangement of elements for electric heating in or on furnaces
    • F27D11/08—Heating by electric discharge, e.g. arc discharge
    • F27D11/10—Disposition of electrodes
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B7/00—Heating by electric discharge
    • H05B7/02—Details
    • H05B7/06—Electrodes

Definitions

  • the invention relates to an electric oven according to the preamble of claim 1.
  • Such an oven is known for example from CH-PS 452 730.
  • the direct current arc furnace In order to optimize the electrical or thermal conditions, it has proven to be advantageous in the direct current arc furnace to form the arc between one or more electrodes arranged above the melting material and the melting material itself. At least one electrode in the bottom of the furnace and in contact with the melt, the bottom electrode, is provided for the return of the direct current.
  • the bottom electrode is exposed to a very high thermal load, for which materials with a high softening and melting point, such as graphite, are suitable.
  • materials with a high softening and melting point such as graphite
  • the melt is carburized on the one hand. However, this is particularly undesirable in the production of low-carbon steels.
  • the carbon electrode is consumed, which can weaken the furnace floor and adversely affect the electrical power transmission.
  • bottom electrodes are used whose zone in connection with the melt also has the same chemical contents as the melt itself.
  • the cooling takes place at the end region of the bottom electrode facing away from the furnace vessel by convection with air, whereby this end region consists of a metal with good heat-conducting and current-carrying properties, for example copper. It is a so-called two-substance base electrode.
  • the heat loss caused by the current in the melting phase results in a greater heat flow in the bottom electrode, specifically in the direction of the furnace bottom.
  • the intensity of the heat available can therefore vary within a relatively wide range between the charging phase and the fresh or refining phase. I.e. but also that the temperatures prevailing in the cooled zone of the bottom electrode also vary within a relatively wide range.
  • the different heat flow in the bottom electrode means, with a constant length of the bottom electrode, a different temperature difference between its cooled zone and the zone in contact with the melt. If there is more heat, there is no higher temperature difference, since the inside of the electrode cannot be warmer than the melt temperature. In other words, more heat can only be transported if the bottom electrode becomes shorter, i.e. H. melts.
  • the temperature of the cooling surface with a greater supply of heat is significantly higher than with a lower supply of heat, the greater amount of heat can only be conducted with an even more shortened length of the base electrode, i.e. H. the bottom electrode melts even more. It follows from this that the change in position of the liquid-solid boundary layer between the melt and the bottom electrode extends over a relatively large length, viewed in the axial direction. This change in position can manifest itself in the waxing of the bottom electrode into the melt, or, as already stated, in a melting process in the direction of the bottom of the furnace vessel.
  • cooler performance must be adapted to the operational requirements. This can be done once can be increased by oversizing an air cooler. In the long run, however, this would lead to unsatisfactory results.
  • liquid cooling is ideal for cooling a bottom electrode.
  • appropriate protective measures must be taken so that liquid metal does not come into contact with the coolant.
  • the invention has for its object to provide an electric furnace of the type mentioned, in whose bottom electrode has a long life. This object is achieved by the invention characterized in claim 1.
  • An essential characteristic of the invention is that the ratio of the cross section of the bottom electrode in the furnace vessel bottom to the cross section in the furnace hearth surface is chosen to be at least 1.4: 1, preferably 2: 1.
  • the undesirably small temperature gradient in the region of the base electrode near the hearth is limited to a small axially extending section, and that the temperature gradient immediately immediately rises relatively steeply in the direction of the furnace vessel bottom.
  • the result of this is that the temperature of the molten bath remains concentrated on the end face of the base electrode in the furnace hearth surface, and that no or only to a very limited extent a change in the position of the liquid / solid boundary layer between the melt and the base electrode can take place. This significantly increases the life of the bottom electrode.
  • At least one molded body supplementing the base electrode is provided, which is made of a refractory material and has a lower conductivity than the base electrode, the shape of the molded body being at least in one Partial area is adapted to the geometric configuration and the shaped body has an increasing cross section in the direction of the interior of the furnace vessel.
  • the molded body encloses the base electrode like a sleeve or is arranged in a funnel-shaped recess which extends from the end face of the base electrode facing the interior of the furnace vessel in the axial direction approximately to the region of the furnace vessel base.
  • the bottom electrode tapers towards the inside of the furnace, it can still be replaced from the outside towards the inside of the furnace. This saves time-consuming and extensive work in the oven vessel cooker that would actually be required to dismantle the bottom electrode in accordance with its geometric shape - namely from the inside out.
  • bottom electrodes made of an iron alloy are used, on the one hand, and two-substance electrodes, on the other hand, in which, on the side facing away from the weld pool, the more electrically and thermally more conductive material, preferably copper, is used.
  • the two metals of the two-substance electrode are preferably welded together.
  • the ratio of the average cross section to the length of the base electrode is equal to a factor multiplied by the electric current.
  • the factor lies in a one-piece iron alloy electrode in a range of 1/200 ⁇ f ⁇ 1/350, and in a two-substance electrode in a range of 1/800 ⁇ f ⁇ 1/1 800.
  • the electrodes become in one area Dimensioned according to the formula above, the floor electrodes are almost exclusively heated by the current alone, with cooling on the side facing away from the inside of the furnace vessel, and on the end face of the bottom electrode facing the furnace hearth surface results in a temperature which corresponds to the temperature of the melting bath.
  • Fig. 1 shows the arc furnace 1 with furnace vessel 2 and furnace lid 3, the furnace vessel 2 from the vessel bottom 4, the vessel wall 5. of the refractory lining of the furnace bottom 4 '. and the refractory lining of the vessel wall 5 '.
  • Above the melt pool 13 is a carbon arranged electrode 8, which protrudes through an opening of the furnace cover 3.
  • a cooling ring 3 ' is arranged for cooling the electrode 8.
  • the electrode 8 is held in a holder 9 of an electrode support arm 11.
  • the electrode support arm 11 is in turn connected to an electrode regulating device, not shown in FIG. 1.
  • the bottom electrode 7a In the vessel bottom 4, 4 ', the bottom electrode 7a according to the invention can be seen, which is enclosed in a sleeve-like manner by the complementary part 10 made of refractory material.
  • the bottom electrode 7a has a conical shape that tapers in the direction of the interior of the vessel, which extends from the bottom 4 of the furnace up to the surface 16 of the furnace.
  • the part 10 which supplements it widens in the direction of the interior of the vessel.
  • the bottom electrode 7a is held below the furnace vessel bottom 4 by a water-cooled connecting piece 17, which is designed as a contact sleeve and which at the same time serves to connect the electrical power supply.
  • the bottom electrode 7a is fastened to the end face of the connection piece 17 by means of a screw connection 23.
  • the bottom electrode 7a lies with its conical contact surfaces on the likewise conical inner contact surfaces of the contact sleeve, which widen towards the furnace bottom 3, whereby a good electrical connection and heat conduction between the two parts 7a and 17 is established.
  • Contact tabs 20 are arranged on the connecting piece 17 and are formed in one piece with the contact sleeve.
  • the connector 17 is provided with cooling channels 19 and with a cooling channel inlet connector 18.
  • a cooling liquid primarily water, is supplied to the cooling channels 19 through the inlet connection 18. It flows upwards through the cooling channels 19 of the connection piece 17 in a spiral arrangement and thus cools the bottom electrode 7a in an indirect manner.
  • the coolant outlet port of the connector 17 is on the same level as the inlet port 18 and is therefore not shown in Fig. 1.
  • the base electrode 7a is held by means of a fastening part, which consists of a metallic frustoconical shielding roof 24 and vertical holding crossbeams 24 ', the shielding roof 24 being arranged at least essentially centrally and open downwards with respect to the furnace axis and by means of the holding crossbeams 24' with the Oven vessel bottom 4 is firmly connected.
  • the bottom electrode 7a projects through the opening of the shielding roof and is supported on the contact sleeve, the connecting piece 17 being fastened to the underside of the shielding roof 24 by inserting an electrically insulating intermediate layer 27.
  • a pin of a pressing device is placed through the opening 28 in the end face 28 of the connecting piece 17 on the end face of the bottom electrode 7a and on the bottom electrode 7a, together with the addition Part 10 applied a force required for the ejection process.
  • the bottom electrode together with the part 10 enveloping it can easily be removed from the outside into the interior of the furnace vessel. Since, in contrast to the end face of the bottom electrode facing the melt, the end face on which the pressing die rests is precisely defined, the pressing out of the bottom electrode from the furnace vessel bottom 4, 4 'can in any case be repeated in a reproducible manner.
  • FIG. 2 shows a vertical section through the base electrode according to the invention in a first exemplary embodiment, according to FIG. 1, but in an enlarged representation.
  • the bottom electrode 7a with the diameter d 1 in the region of the furnace vessel bottom 4 tapers conically up to the hearth surface 16 and has a diameter d 3 there.
  • the dashed line 10 ' indicates that the part 10 could also be cylindrical, without the pressing-out process of the base electrode from the furnace base 5 being thereby made significantly more difficult.
  • the average diameter of the bottom electrode 7a is denoted by d 2 and the total length of the bottom electrode 7a by I.
  • the bottom electrode 7b has a cylindrical shape and is likewise encased with the additional part 10, which in turn widens in the direction of the interior of the vessel. In this way, the pressing out of the bottom electrode 7b from the bottom of the furnace vessel is in turn made considerably easier.
  • the bottom electrode 7b - as can be seen from the dashed line - has a shaped body 15 made of a refractory material, which is arranged in a funnel-shaped recess within the bottom electrode 7b, the recess extending from the end face of the bottom electrode 7b facing the interior of the vessel in the axial direction extends to the area of the furnace vessel bottom 4.
  • the shaped body 15 serves for the purpose of reducing the cross-section of the bottom electrode 7b in the direction of the interior of the furnace, the ratio of the cross-section of the bottom electrode 7a, b, c, d in the bottom 4 of the furnace to the cross-section in the furnace surface 16 being at least 1.4: 1. preferably 2: 1 was chosen.
  • the liquid / solid boundary layer between the melt pool 13 and the end face the bottom electrode 7a, b, c, d in the furnace hearth surface is kept very stable and the service life of the bottom electrode 7a, b, c, d and the refractory lining 4 'of the furnace vessel bottom in the region close to the electrodes are significantly increased.
  • the power loss of the bottom electrode 7a, b, c, d can be reduced to a minimum.
  • the average cross section results from the arithmetic mean of the cross section of the bottom electrode 7a, b, c, d in the area of the furnace vessel bottom 4 and the cross section in the furnace hearth surface 16.
  • the electrode 7b has a diameter of d 1 , which is reduced by the shaped body 15 in the furnace hearth surface 16 up to the radial ring width d s .
  • the mean diameter is denoted by d 6 and the length is again denoted by I.
  • FIG. 4 shows the bottom electrode 7c, the outer diameter of which widens towards the interior of the vessel or at most remains the same, as indicated by the dashed lines 7c '.
  • the bottom electrode 7c has no part that complements it, because the squeezing out to the inside of the vessel is ensured even without it.
  • the bottom electrode 7c is provided with a funnel-shaped shaped body 15 which, as already explained in the description of FIG. 3, stabilizes the liquid / solid boundary layer between the molten bath 13 and the end face of the bottom electrode 7a, b, c, d and Power loss limitation.
  • the bottom electrode 7c has a diameter d 1 in the bottom 4 of the furnace. in the oven surface 16 an outer diameter d 8 , but only a metallic ring width d lo in the oven surface 16.
  • the mean diameter is designated with dg, the length with I.
  • FIG. 5 shows a two-substance base electrode 7d, which consists of an iron alloy part 31 facing the furnace hearth surface 16 and the copper part 32. Both parts 31, 32 are metallurgically connected to one another, with the electrode 7b being pressed out from the outside in the direction of the interior of the vessel.
  • the exemplary embodiments of the bottom electrode 7a, b, c shown in FIGS. 2 to 4 have the following temperature profiles:
  • Temperature profile D The temperature gradient in the section 30 increases in the area near the oven compared to the electrode 7b with a cylindrical cross-sectional shape, ie a melting process in section 30 is largely suppressed.
  • the temperature of the bottom electrodes 7a, b, c - temperature profile D - approaches the temperature profile F, that is to say the temperature of the part 10, 10 'made of refractory material or the refractory lining 4' which supplements and surrounds the bottom electrode 7a, b. of the furnace vessel bottom 4. This is of crucial importance for an electric furnace that is used in continuous industrial operation.
  • Temperature profile B The temperature gradient in section 30 runs very steeply, the boundary layer liquid / solid between molten bath 13 and end face of bottom electrodes 7a, b, c remains stable in furnace hearth surface 16.
  • bottom electrodes 7a, b, c the cross-section of which in the oven hearth surface 16 tapers in relation to the oven vessel bottom 4, specifically in accordance with the dimensioning ratio according to the invention, the temperature gradient in the section 30 of the bottom electrodes 7a, b, c near the hearth surface can be increased . It is thereby achieved that the position of the liquid / solid boundary layer between the melt pool 13 and the end face of the bottom electrodes 7a, b, c can largely be located in the furnace hearth area 16. The bottom electrode 7a, b, c is not melted off and its service life is significantly increased. In addition, this measure ensures that the bottom electrode 7a, b, c can be better dimensioned for minimal power loss.
  • the temperature fluctuations of the bottom electrodes 7a, b, c which result from the states with and without current flow, are closer to the temperature of the refractory lining of the furnace vessel bottom 4 surrounding the bottom electrodes 7a, b, c.
  • FIG. 7 finally shows the temperature profile of a two-substance base electrode according to FIG. 5, but with a cross section that slightly decreases inwards.
  • I again indicates the temperature of the bottom electrode 7d in the furnace hearth surface 16, and A the temperature in the cooled section. This reduction in cross-section is achieved by attaching a shaped body 15 within the bottom electrode 7d. This molded body 15 was not shown in FIG. 5.
  • G 1 shows the temperature profile of the part made of iron alloy
  • G 2 that of the part made of copper
  • H represents the temperature of the refractory lining 4 'of the furnace bottom 4.
  • the temperature profile G 1 and G 2 approximates the temperature profile H. This shows the effect of the poorly thermally conductive iron alloy, despite the relatively large cross-section.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Plasma & Fusion (AREA)
  • Vertical, Hearth, Or Arc Furnaces (AREA)
  • Discharge Heating (AREA)

Abstract

1. Electric furnace, in particular a direct-current arc furnace for melting metals, with at least one bottom electrode (7a, b, c, d) which is cooled on its side facing away from the interior of the furnace vessel, characterized in that the ratio of the cross-section of the bottom electrode (7a, b, c, d) in the furnace vessel bottom (4) to the cross-section in the furnace hearth area (16) is selected to be greater than 1.4 : 1, preferably 2 : 1.

Description

Die Erfindung bezieht sich auf einen elektrischen Ofen nach dem Gattungsbegriff des Patentanspruchs 1. Ein derartiger Ofen ist beispielsweise aus der CH-PS 452 730 bekannt.The invention relates to an electric oven according to the preamble of claim 1. Such an oven is known for example from CH-PS 452 730.

Die Fortschritte in der Entwicklung von Halbleiterbauelementen in den vergangenen Jahren waren Anlass dafür, Gleichstromlichtbogenöfen in zunehmendem Ausmass in der Eisen- und Stahlindustrie zur Erschmelzung, vornehmlich von Elektrostahl, einzusetzen.The advances in the development of semiconductor components in recent years have prompted DC arc furnaces to be used to an increasing extent in the iron and steel industry for melting, primarily of electrical steel.

Aufbau und Wirkungsweise von Gleichstromlichtbogenöfen sind beispielsweise aus der Zeitschrift « Stahl und Eisen •, 103 (1983) Nr. 3, vom 14. Februar 1983, Seiten 133 bis 137 bekannt.The structure and mode of operation of direct current arc furnaces are known, for example, from the journal “Stahl und Eisen •, 103 (1983) No. 3, from February 14, 1983, pages 133 to 137.

Zur Optimierung der elektrischen bzw. thermischen Verhältnisse hat es sich beim Gleichstromlichtbogenofen als vorteilhaft erwiesen, den Lichtbogen zwischen einer oder mehreren oberhalb des Schmelzgutes angeordneten Elektrode(n) und dem Schmelzgut selbst auszubilden. Für die Rückleitung des Gleichstromes ist mindestens eine im Boden des Ofens und mit der Schmelze in Berührung stehende Elektrode, die Bodenelektrode, vorgesehen.In order to optimize the electrical or thermal conditions, it has proven to be advantageous in the direct current arc furnace to form the arc between one or more electrodes arranged above the melting material and the melting material itself. At least one electrode in the bottom of the furnace and in contact with the melt, the bottom electrode, is provided for the return of the direct current.

Die Bodenelektrode ist einer anhaltenden sehr hohen thermischen Beanspruchung ausgesetzt, für welche sich Werkstoffe mit einem hohen Erweichungs- und Schmelzpunkt, beispielsweise Graphit, eignen. Bei Verwendung von Kohlenstoffelektroden wird aber die Schmelze einerseits aufgekohlt. Dies ist jedoch insbesondere bei der Herstellung von niedrig gekohlten Stählen unerwünscht. Andererseits wird die Kohlenstoffelektrode aufgezehrt, wodurch der Ofenboden geschwächt und die elektrische Leistungsübertragung ungünstig beeinflusst werden kann.The bottom electrode is exposed to a very high thermal load, for which materials with a high softening and melting point, such as graphite, are suitable. When using carbon electrodes, however, the melt is carburized on the one hand. However, this is particularly undesirable in the production of low-carbon steels. On the other hand, the carbon electrode is consumed, which can weaken the furnace floor and adversely affect the electrical power transmission.

Nach dem Lösungsvorschlag der CH-PS 452 730 werden Bodenelektroden verwendet, deren mit der Schmelze in Verbindung stehende Zone auch dieselben chemischen Gehalte aufweist, wie die Schmelze selbst. Die Kühlung erfolgt dabei an dem dem Ofengefäss abgewandten Endbereich der Bodenelektrode durch Konvektion mit Luft, wobei dieser Endbereich aus einem Metall mit guten wärmeleitenden und stromführenden Eigenschaften, beispielsweise kupfer, besteht. Es handelt sich hierbei um eine sogenannte Zweistoff-Bodenelektrode.According to the proposed solution in CH-PS 452 730, bottom electrodes are used whose zone in connection with the melt also has the same chemical contents as the melt itself. The cooling takes place at the end region of the bottom electrode facing away from the furnace vessel by convection with air, whereby this end region consists of a metal with good heat-conducting and current-carrying properties, for example copper. It is a so-called two-substance base electrode.

Mit dieser luftgekühlten Zweistoff-Bodenelektrode vermeidet man einerseits, dass bei einem eventuellen Durchbruch des Ofenherdes flüssiges Metall mit Flüssigkeit einer Kühlanordnung oder stromführenden Bauteilen der Bodenelektrode unterhalb des Ofengefässbodens in Kontakt treten kann und schaltet so von vornherein die Gefahr unvorhergesehener schwerwiegender Folgen aus. Andererseits muss aber ein relativ schwacher Kühleffekt in Kauf genommen werden, der den Anforderungen, die an eine Bodenelektrode im Dauerbetrieb des industriellen Einsatzes gestellt werden, keineswegs gewachsen ist, und zwar aus folgenden Gründen :

  • Der Betrieb eines Lichtbogenofens ist im wesentlichen durch fünf Verfahrensabläufe charakterisiert :
    • - die Chargierphase ohne Strom ; tiefe Temperatur
    • - die Einschmelzphase hoher Strom ; hohe Temperatur
    • - die Frisch- oder Feinungsphase kleiner Strom ; hohe Temperatur
    • - die Abstechphase ohne Strom ; hohe Temperatur
    • - die Nebenzeitphasen ohne Strom ; mittlere-tiefe Temperatur.
With this air-cooled two-substance floor electrode, one prevents on the one hand that in the event of a breakthrough in the furnace hearth, liquid metal can come into contact with liquid from a cooling arrangement or current-carrying components of the floor electrode underneath the furnace bottom, thus eliminating the risk of unforeseen serious consequences from the outset. On the other hand, however, a relatively weak cooling effect has to be accepted, which is in no way up to the demands placed on a floor electrode in continuous operation in industrial use, for the following reasons:
  • The operation of an arc furnace is essentially characterized by five procedures:
    • - the charging phase without electricity; low temperature
    • - the melting phase of high current; high temperature
    • - the fresh or refining phase of small stream; high temperature
    • - the cut-off phase without electricity; high temperature
    • - the off-peak phases without electricity; medium-low temperature.

Insbesondere die in der Einschmelzphase entstehende Verlustwärme durch den Strom hat einen grösseren Wärmefluss in der Bodenelektrode und zwar in Richtung Ofengefässboden zur Folge. Zwischen der Chargierphase und Frisch- oder Feinungsphase kann demnach die Intensität des Wärmeangebotes in einem relativ weiten Bereich variieren. D. h. aber auch, dass die in der gekühlten Zone der Bodenelektrode herrschenden Temperaturen ebenfalls in einem relativ weiten Bereich variieren. Der unterschiedliche Wärlmefluss in der Bodenelektrode bedeutet bei konstanter Länge der Bodenelektrode eine unterschiedliche Temperaturdifferenz zwischen deren gekühlter und mit der Schmelze in Berührung stehender Zone. Bei grösserem Wärmeangebot ist aber keine höhere Temperaturdifferenz vorhanden, da die Elektrode auf der Innenseite nicht wärmer sein kann als die Schmelzentemperatur. Oder anders ausgedrückt, es kann mehr Wärme nur dann transportiert werden, wenn die Bodenelektrode kürzer wird, d. h. abschmiltz.In particular, the heat loss caused by the current in the melting phase results in a greater heat flow in the bottom electrode, specifically in the direction of the furnace bottom. The intensity of the heat available can therefore vary within a relatively wide range between the charging phase and the fresh or refining phase. I.e. but also that the temperatures prevailing in the cooled zone of the bottom electrode also vary within a relatively wide range. The different heat flow in the bottom electrode means, with a constant length of the bottom electrode, a different temperature difference between its cooled zone and the zone in contact with the melt. If there is more heat, there is no higher temperature difference, since the inside of the electrode cannot be warmer than the melt temperature. In other words, more heat can only be transported if the bottom electrode becomes shorter, i.e. H. melts.

Wenn nun, wie es bei Luftkühlung in einem weiten Bereich der Fall ist, die Temperatur der Kühlfläche bei grösserem Wärmeangebot wesentlich höher ist als bei geringerem Wärmeangebot, so kann die grössere Wärmemenge nur bei noch stärker verkürzter Länge der Bodenelektrode geführt werden, d. h. die Bodenelektrode schmilzt noch mehr ab. Daraus folgt, dass sich die Lageänderung der Grenzschicht Flüssig-Fest zwischen Schmelze und Bodenelektrode über eine relativ grosse Länge - in axialer Richtung betrachtet - erstreckt. Diese Lageänderung kann sich einmal in einem « Hineinwachsen der Bodenelektrode in die Schmelze, oder, wie bereits ausgeführt, in einem Abschmelzvorgang in Richtung Ofengefässboden, äussern.If, as is the case with air cooling in a wide range, the temperature of the cooling surface with a greater supply of heat is significantly higher than with a lower supply of heat, the greater amount of heat can only be conducted with an even more shortened length of the base electrode, i.e. H. the bottom electrode melts even more. It follows from this that the change in position of the liquid-solid boundary layer between the melt and the bottom electrode extends over a relatively large length, viewed in the axial direction. This change in position can manifest itself in the waxing of the bottom electrode into the melt, or, as already stated, in a melting process in the direction of the bottom of the furnace vessel.

Dieser vorstehend geschilderte Vorgang beeinträchtigt die Haltbarkeit der Bodenelektrode in erheblichem Mass und führt zu einer frühzeitigen Zerstörung der die Bodenelektrode umgebenden feuerfesten Auskleidung des Ofengefässbodens. Damit die Bodenelektrode unter derart ungünstigen Betriebsbedingungen überhaupt funktionsfähig ist, muss sie entsprechend überdimensioniert werden. Dies wiederum beeinflusst die Verlustleistung in nachteiliger Weise.This process described above significantly affects the durability of the base electrode and leads to premature destruction of the refractory lining of the furnace vessel base surrounding the base electrode. So that the bottom electrode is functional at all under such unfavorable operating conditions, it must be oversized accordingly. This in turn adversely affects the power loss.

Darüber hinaus ist Kühlerleistung den betrieblichen Erfordernissen anzupassen. Diese kann einmal erhöht werden, durch eine Ueberdimensionierung eines Luftkühlers. Dies würde jedoch auf die Dauer zu unbefriedigenden Ergebnissen führen.In addition, the cooler performance must be adapted to the operational requirements. This can be done once can be increased by oversizing an air cooler. In the long run, however, this would lead to unsatisfactory results.

Andererseits bietet sich eine Flüssigkeitskühlung in hervorragender Weise für die Kühlung einer Bodenelektrode an. Hierbei müssen aber entsprechende Schutzvorkehrungen getroffen werden, damit flüssiges Metall nicht mit Kühlflüssigkeit in Berührung kommt.On the other hand, liquid cooling is ideal for cooling a bottom electrode. However, appropriate protective measures must be taken so that liquid metal does not come into contact with the coolant.

Der Erfindung liegt die Aufgabe zugrunde, einen elektrischen Ofen der eingangs genannten Art zu schaffen, bei dessen Bodenelektrode eine hohe Lebensdauer aufweist. Die Lösung dieser Aufgabe erfolgt durch die im Patentanspruch 1 gekennzeichnet Erfindung.The invention has for its object to provide an electric furnace of the type mentioned, in whose bottom electrode has a long life. This object is achieved by the invention characterized in claim 1.

Wesentliches Kennzeichen der Erfindung ist, dass das Verhältnis des Querschnittes der Bodenelektrode im Ofengefässboden zum Querschnitt in der Ofenherdfläche mindestens 1,4 : 1, vorzugsweise 2 : 1 gewählt ist. Dadurch wird erreicht, dass der im herdflächennahen Bereich der Bodenelektrode unerwünscht kleine Temperaturgradient auf einen kleinen axial sich erstreckenden Abschnitt beschränkt bleibt, und dass an diesen Bereich in Richtung Ofengefässboden unmittelbar angrenzend der Temperaturgradient sofort relativ steil ansteigt. Das hat zur Folge, dass die Temperatur des Schmelzbades auf der Stirnfläche der Bodenelektrode in der Ofenherdfläche konzentriert bleibt, und dass keine oder nur in einem sehr beschränkten Ausmass eine Lageänderung der Grenzschicht Flüssig/Fest zwischen Schmelze und Bodenelektrode sich vollziehen kann. Dadurch wird die Lebensdauer der Bodenelektrode wesentlich erhöht.An essential characteristic of the invention is that the ratio of the cross section of the bottom electrode in the furnace vessel bottom to the cross section in the furnace hearth surface is chosen to be at least 1.4: 1, preferably 2: 1. The result of this is that the undesirably small temperature gradient in the region of the base electrode near the hearth is limited to a small axially extending section, and that the temperature gradient immediately immediately rises relatively steeply in the direction of the furnace vessel bottom. The result of this is that the temperature of the molten bath remains concentrated on the end face of the base electrode in the furnace hearth surface, and that no or only to a very limited extent a change in the position of the liquid / solid boundary layer between the melt and the base electrode can take place. This significantly increases the life of the bottom electrode.

Bei der Weiterbildung des Erfindungsgegenstandes gemäss den Ansprüchen 2, 3 und 4 ist mindestens ein die Bodenelektrode zu einer Einheit ergänzender Formkörper vorgesehen, der aus einem feuerfesten Werkstoff besteht und eine im Vergleich zur Bodenelektrode geringere Leitfähigkeit aufweist, wobei der Formkörper in seiner Formgebung zumindest in einem Teilbereich an die geometrische Ausgestaltung angepasst ist und der Formkörper in Richtung des Ofengefässinneren einen zunehmenden querschnitt aufweist. Der Formkörper umschliesst einmal die Bodenelektrode hülsenartig oder ist andererseits in einer trichterförmigen Ausnehmung angeordnet, welche sich von der dem Ofengefässinneren zugewandten Stirnfläche der Bodenelektrode in axialer Richtung sich annähernd bis zum Bereich des Ofengefässbodens erstreckt.In the development of the subject matter of the invention according to claims 2, 3 and 4, at least one molded body supplementing the base electrode is provided, which is made of a refractory material and has a lower conductivity than the base electrode, the shape of the molded body being at least in one Partial area is adapted to the geometric configuration and the shaped body has an increasing cross section in the direction of the interior of the furnace vessel. The molded body encloses the base electrode like a sleeve or is arranged in a funnel-shaped recess which extends from the end face of the base electrode facing the interior of the furnace vessel in the axial direction approximately to the region of the furnace vessel base.

Auf diese Weise kann, obwohl sich die Bodenelektrode in Richtung Ofengefässinneres verjüngt, diese dennoch von Aussen in Richtung Ofengefässinneres ausgewechselt werden. Dadurch werden aufwendige und umfangreiche Arbeiten im Ofengefässherd eingespart, die zur Demontage der Bodenelektrode entsprechend ihrer geometrischen Form - nämlich von Innen nach Aussen - eigentlich erforderlich wären.In this way, although the bottom electrode tapers towards the inside of the furnace, it can still be replaced from the outside towards the inside of the furnace. This saves time-consuming and extensive work in the oven vessel cooker that would actually be required to dismantle the bottom electrode in accordance with its geometric shape - namely from the inside out.

Bei der Weiterbildung der Erfindung werden einerseits Bodenelektroden aus einer Eisenlegierung verwendet und andererseits Zweistoffelektroden, bei der auf der dem Schmelzbad abgewandten Seite der elektrisch und thermisch besser leitfähige Werkstoff, und zwar vorzugsweise Kupfer verwendet wird. Die beiden Metalle der Zweistoffelektrode sind vorzugsweise miteinander verschweisst. Durch die Anwendung zweier unterschiedlicher Werkstoffe in genannter Weise wird eine zusätzliche Einengung des Bereiches mit kleinem Temperaturgradienten in der Stirnfläche erreicht und dadurch wird das Temperaturprofil in der Bodenelektrode demjenigen des Ofenherdes weitgehend angenähert.In the development of the invention, bottom electrodes made of an iron alloy are used, on the one hand, and two-substance electrodes, on the other hand, in which, on the side facing away from the weld pool, the more electrically and thermally more conductive material, preferably copper, is used. The two metals of the two-substance electrode are preferably welded together. By using two different materials in the above-mentioned manner, an additional narrowing of the area with a small temperature gradient in the end face is achieved, and the temperature profile in the bottom electrode is thus largely approximated to that of the oven range.

Bei der Weiterbildung der Erfindung ist das Verhältnis von mittlerem Querschnitt zur Länge der Bodenelektrode gleich einem Faktor multipliziert mit dem elektrischen Strom. Dabei liegt der Faktor bei einer einstückig ausgebildeten Elektrode aus Eisenlegierung in einem Bereich von 1/200 < f < 1/350, und bei einer Zweistoffelektrode in einem Bereich von 1/800 < f < 1/1 800. Werden die Elektroden in einem Bereich gemäss oben stehender Formel dimensioniert, so erfolgt annähernd die Erwärmung der Bodenelektroden allein durch den Strom, bei Kühlung an der dem Ofengefässinneren abgewandten Seite, und ergibt an der der Ofenherdfläche zugewandten Stirnfläche der Bodenelektrode eine Temperatur, die der Temperatur des Schmelzbades entspricht.In the development of the invention, the ratio of the average cross section to the length of the base electrode is equal to a factor multiplied by the electric current. The factor lies in a one-piece iron alloy electrode in a range of 1/200 <f <1/350, and in a two-substance electrode in a range of 1/800 <f <1/1 800. The electrodes become in one area Dimensioned according to the formula above, the floor electrodes are almost exclusively heated by the current alone, with cooling on the side facing away from the inside of the furnace vessel, and on the end face of the bottom electrode facing the furnace hearth surface results in a temperature which corresponds to the temperature of the melting bath.

Die Erfindung wird nachstehend anhand von Ausführungsbeispielen in der Zeichnung näher erläutert.The invention is explained in more detail below on the basis of exemplary embodiments in the drawing.

Es zeigt :

  • Figur 1 einen Vertikalschnitt durch den elektrischen Ofen mit der erfindungsgemässen Bodenelektrode,
  • Figur 2 einen Vertikalschnitt durch die erfindungsgemässe Bodenelektrode in einem ersten Ausführungsbeispiel gemäss Fig. 1, jedoch in vergrösserter Darstellung,
  • Figur 3 einen Vertikalschnitt durch die erfindungsgemässe Bodenelektrode in einem weiteren Ausführungsbeispiel,
  • Figur 4 einen Vertikalschnitt durch die erfindungsgemässe Bodenelektrode in einem weiteren Ausführungsbeispiel,
  • Figur 5 einen Vertikalschnitt durch die erfindungsgemässe Zweistoff-Bodenelektrode,
  • Figur 6 Schaubild des Temperaturprofils einer einstückig ausgebildeten Bodenelektrode aus Eisenlegierung mit verschiedenen geometrischen Ausbildungsformen, und
  • Figur 7 Schaubild des Temperaturprofiles einer Zweistoff-Bodenelektrode.
It shows :
  • FIG. 1 shows a vertical section through the electric furnace with the base electrode according to the invention,
  • FIG. 2 shows a vertical section through the base electrode according to the invention in a first exemplary embodiment according to FIG. 1, but in an enlarged view,
  • FIG. 3 shows a vertical section through the base electrode according to the invention in a further exemplary embodiment,
  • FIG. 4 shows a vertical section through the base electrode according to the invention in a further exemplary embodiment,
  • FIG. 5 shows a vertical section through the two-substance base electrode according to the invention,
  • FIG. 6 graph of the temperature profile of a one-piece base electrode made of iron alloy with different geometrical shapes, and
  • FIG. 7 graph of the temperature profile of a two-substance base electrode.

Fig. 1 zeigt den Lichtbogenofen 1 mit Ofengefäss 2 und Ofendeckel 3, wobei das Ofengefäss 2 aus dem Gefässboden 4, der Gefässwand 5. der feuerfesten Auskleidung des Ofenbodens 4'. sowie der feuerfesten Auskleidung der Gefässwand 5' besteht. Oberhalb des Schmelzbades 13 ist eine Kohlenstoffelektrode 8 angeordnet, welche durch eine Oeffnung des Ofendeckels 3 hindurchragt. Zur Kühlung der Elektrode 8 ist ein Kühlring 3' angeordnet. Die Elektrode 8 ist in einer Halterung 9 eines Elektrodentragarmes 11 gehalten. Der Elektrodentragarm 11 ist wiederum mit einer in Fig. 1 nicht dargestellten Elektrodenreguliereinrichtung verbunden.Fig. 1 shows the arc furnace 1 with furnace vessel 2 and furnace lid 3, the furnace vessel 2 from the vessel bottom 4, the vessel wall 5. of the refractory lining of the furnace bottom 4 '. and the refractory lining of the vessel wall 5 '. Above the melt pool 13 is a carbon arranged electrode 8, which protrudes through an opening of the furnace cover 3. A cooling ring 3 'is arranged for cooling the electrode 8. The electrode 8 is held in a holder 9 of an electrode support arm 11. The electrode support arm 11 is in turn connected to an electrode regulating device, not shown in FIG. 1.

In dem Ofengefäss 5, 5' befindet sich eine Ofentür 6 und zwischen der Elektrode 8 und dem Schmelzbad 13 ist ein Lichtbogen 14 ausgebildet.There is an oven door 6 in the furnace vessel 5, 5 ′ and an arc 14 is formed between the electrode 8 and the molten bath 13.

Im Gefässboden 4, 4' ist die erfindungsgemässe Bodenelektrode 7a zu sehen, welche von dem sie ergänzenden Teil 10 aus feuerfestem Werkstoff hülsenartig umschlossen ist. In der beispielsweisen Ausführungsform gemäss Fig. 1 weist die Bodenelektrode 7a eine konische, sich in Richtung des Gefässinneren verjüngende Form auf, welche sich vom Ofengefässboden 4 bis zur Ofenherdfläche 16 erstreckt. Im Gegensatz zur sich verjüngenden Gestalt der Bodenelektrode 7a, erweitert sich der sie ergänzende Teil 10 in Richtung des Gefässinneren hin. Die Bodenelektrode 7a wird unterhalb des Ofengefässbodens 4 durch ein wassergekühltes, als Kontakthülse ausgebildetes Anschlussstück 17 gehalten, welches gleichzeitig zur Verbindung der elektrischen Stromzuführung dient. Die Bodenelektrode 7a ist mittels einer Schraubverbindung 23 an der Stirnfläche des Anschlussstückes 17 befestigt. Die Bodenelektrode 7a liegt mit ihren konisch ausgebildeten Kontaktflächen an den ebenfalls konischen, zum Ofenboden 3 hin sich erweiternden inneren Kontaktflächen der Kontakthülse an, wodurch eine gute elektrische Verbindung und Wärmeleitung zwischen beiden Teilen 7a und 17 hergestellt wird. An dem Anschlussstück 17 sind Kontaktlaschen 20 angeordnet, die einstückig mit der Kontakthülse ausgebildet sind.In the vessel bottom 4, 4 ', the bottom electrode 7a according to the invention can be seen, which is enclosed in a sleeve-like manner by the complementary part 10 made of refractory material. In the exemplary embodiment according to FIG. 1, the bottom electrode 7a has a conical shape that tapers in the direction of the interior of the vessel, which extends from the bottom 4 of the furnace up to the surface 16 of the furnace. In contrast to the tapered shape of the bottom electrode 7a, the part 10 which supplements it widens in the direction of the interior of the vessel. The bottom electrode 7a is held below the furnace vessel bottom 4 by a water-cooled connecting piece 17, which is designed as a contact sleeve and which at the same time serves to connect the electrical power supply. The bottom electrode 7a is fastened to the end face of the connection piece 17 by means of a screw connection 23. The bottom electrode 7a lies with its conical contact surfaces on the likewise conical inner contact surfaces of the contact sleeve, which widen towards the furnace bottom 3, whereby a good electrical connection and heat conduction between the two parts 7a and 17 is established. Contact tabs 20 are arranged on the connecting piece 17 and are formed in one piece with the contact sleeve.

In Fig. 1 ist ein Teil des elektrischen Stromzuführungskabels 22 zu sehen, das mittels Schraubverbindung 21 mit den Kontaktlaschen 20 verbunden ist. Das Anschlussstück 17 ist mit Kühlkanälen 19 sowie mit einem Kühlkanaleintrittsstutzen 18 versehen. Eine Kühlflüssigkeit, vornehmlich Wasser, wird durch den Eintrittsstutzen 18 den Kühlkanälen 19 zugeführt. Es durchströmt die Kühlkanäle 19 des Anschlussstückes 17 in spiralförmiger Anordnung aufwärts und kühlt somit die Bodenelektrode 7a auf indirekte Weise. Der Kühlflüssigkeitsaustrittsstutzen des Anschlussstückes 17 befindet sich auf der gleichen Ebene wie der Eintrittsstutzen 18 und ist deshalb in Fig. 1 nicht zu sehen.1 shows a part of the electrical power supply cable 22 which is connected to the contact tabs 20 by means of a screw connection 21. The connector 17 is provided with cooling channels 19 and with a cooling channel inlet connector 18. A cooling liquid, primarily water, is supplied to the cooling channels 19 through the inlet connection 18. It flows upwards through the cooling channels 19 of the connection piece 17 in a spiral arrangement and thus cools the bottom electrode 7a in an indirect manner. The coolant outlet port of the connector 17 is on the same level as the inlet port 18 and is therefore not shown in Fig. 1.

Die Halterung der Bodenelektrode 7a geschieht mittels eines Befestigungsteiles, welche aus einem metallischen kegelstumpfartigen Abschirmdach 24 und vertikalen Haltetraversen 24' besteht, wobei das Abschirmdach 24 in bezug auf die Ofenachse mindestens im wesentlichen zentral und nach unten offen angeordnet und mittels der Haltetraversen 24' mit dem Ofengefässboden 4 fest verbunden ist. Die Bodenelektrode 7a ragt durch die Oeffnung des Abschirmdaches hindurch und stützt sich auf der Kontakthülse ab, wobei das Anschlussstück 17 unter Einfügung einer elektrisch isolierenden Zwischenschicht 27 an der Unterseite des Abschirmdaches 24 befestigt ist.The base electrode 7a is held by means of a fastening part, which consists of a metallic frustoconical shielding roof 24 and vertical holding crossbeams 24 ', the shielding roof 24 being arranged at least essentially centrally and open downwards with respect to the furnace axis and by means of the holding crossbeams 24' with the Oven vessel bottom 4 is firmly connected. The bottom electrode 7a projects through the opening of the shielding roof and is supported on the contact sleeve, the connecting piece 17 being fastened to the underside of the shielding roof 24 by inserting an electrically insulating intermediate layer 27.

Bei einer allfälligen Demontage der Bodenelektrode wird nur die Verschraubung 23 gelöst. An die, dem Gefässinneren abgewandten Stirnfläche der Bodenelektrode 7a wird beispielsweise ein in Fig. 1 nicht dargestellter Bolzen einer Auspressvorrichtung durch den Durchbruch 28 in der Stirnfläche 28 des Anschlussstückes 17 auf die Stirnfläche der Bodenelektrode 7a aufgesetzt und auf die Bodenelektrode 7a, mitsamt dem sie ergänzenden Teil 10 eine für den Ausstossvorgang erforderliche Kraft aufgebracht. Auf diese Weise kann die Bodenelektrode mitsamt dem sie umhüllenden Teil 10 leicht von aussen in das Ofengefässinnere hinein entfernt werden. Da im Gegensatz zu der der Schmelze zugewandten Stirnfläche der Bodenelektrode die Stirnfläche, auf der der Auspressstempel anliegt, genau definiert ist, so kann die Auspressung der Bodenelektrode aus dem Ofengefässboden 4, 4' in jedem Fall gut reproduzierbar wiederholt werden.If the base electrode is removed, only the screw connection 23 is loosened. On the end face of the bottom electrode 7a facing away from the interior of the vessel, for example, a pin of a pressing device, not shown in FIG. 1, is placed through the opening 28 in the end face 28 of the connecting piece 17 on the end face of the bottom electrode 7a and on the bottom electrode 7a, together with the addition Part 10 applied a force required for the ejection process. In this way, the bottom electrode together with the part 10 enveloping it can easily be removed from the outside into the interior of the furnace vessel. Since, in contrast to the end face of the bottom electrode facing the melt, the end face on which the pressing die rests is precisely defined, the pressing out of the bottom electrode from the furnace vessel bottom 4, 4 'can in any case be repeated in a reproducible manner.

Fig. 2 zeigt einen Vertikalschnitt durch die erfindungsgemässe Bodenelektrode in einem ersten Ausführungsbeispiel, gemäss Fig. 1, jedoch in vergrösserter Darstellung. Die Bodenelektrode 7a mit dem Durchmesser d1 im Bereich des Ofengefässbodens 4 verjüngt sich konisch bis zur Herdfläche 16 hin und weist dort einen Durchmesser d3 auf. Der die Bodenelektrode 7a ergänzende Teil 10 hingegen, welcher im Bereich des Ofenbodens 4 annähernd ebenfalls einen Durchmesser d1 aufweist, erweitert sich und hat in der Herdfläche den Durchmesser d4. Durch die strichlierte Linie 10' ist angedeutet, dass der Teil 10 ebenfalls zylindrisch ausgebildet sein könnte, ohne dass der Auspressvorgang der Bodenelektrode aus dem Ofenboden 5 dadurch wesentlich erschwert würde. Der mittlere Durchmesser der Bodenelektrode 7a ist mit d2 bezeichnet und die Gesamtlänge der Bodenelektrode 7a mit I.FIG. 2 shows a vertical section through the base electrode according to the invention in a first exemplary embodiment, according to FIG. 1, but in an enlarged representation. The bottom electrode 7a with the diameter d 1 in the region of the furnace vessel bottom 4 tapers conically up to the hearth surface 16 and has a diameter d 3 there. The part 10 which supplements the base electrode 7a, on the other hand, which also has a diameter d 1 in the region of the furnace base 4, widens and has the diameter d 4 in the hearth surface. The dashed line 10 'indicates that the part 10 could also be cylindrical, without the pressing-out process of the base electrode from the furnace base 5 being thereby made significantly more difficult. The average diameter of the bottom electrode 7a is denoted by d 2 and the total length of the bottom electrode 7a by I.

Fig. 3 zeigt einen Vertikalschnitt durch eine weitere erfindungsgemässe Bodenelektrode 7b. Die Bodenelektrode 7b weist eine zylindrische Gestalt auf und ist gleichfalls mit dem ergänzenden Teil 10 umhüllt, der sich wiederum in Richtung des Gefässinneren hin, erweitert. Auf diese Weise wird die Auspressung der Bodenelektrode 7b aus dem Ofengefässboden wiederum wesentlich erleichtert. Die Bodenelektrode 7b weist - durch die strichlierte Linie ersichtlich - einen Formkörper 15 aus einem feuerfesten Werkstoff auf, welcher in einer trichterförmigen Ausnehmung innerhalb der Bodenelektrode 7b angeordnet ist, wobei sich die Ausnehmung von der dem Gefässinneren zugewandten Stirnfläche der Bodenelektrode 7b in axialer Richtung sich bis zum Bereich des Ofengefässbodens 4 erstreckt. Der Formkörper 15 dient zum Zweck der Querschnittsverringerung der Bodenelektrode 7b in Richtung Ofengefässinneres, wobei das Verhältnis des Querschnittes der Bodenelektrode 7a, b, c, d im Ofengefässboden 4 zum Querschnitt in der Ofenherdfläche 16 mindestens 1.4:1. vorzugsweise 2 : 1 gewählt wurde. Dadurch kann die Grenzschicht Flüssig/Fest zwischen Schmelzbad 13 und Stirnfläche der Bodenelektrode 7a, b, c, d in der Ofenherdfläche sehr lagestabil gehalten und die Lebensdauer der Bodenelektrode 7a, b, c, d und die feuerfeste Auskleidung 4' des Ofengefässbodens im elektrodennahen Bereich wesentlich erhöht werden. Wird der Querschnitt und die Länge der Bodenelektrode 7a, b, c, d dem sie durchfliessenden Strom unter zusätzlicher Berücksichtigung eines Faktors angepasst, so kann gleichzeitig die Verlustleistung der Bodenelektrode 7a, b, c, d auf ein Minimum reduziert werden.3 shows a vertical section through a further bottom electrode 7b according to the invention. The bottom electrode 7b has a cylindrical shape and is likewise encased with the additional part 10, which in turn widens in the direction of the interior of the vessel. In this way, the pressing out of the bottom electrode 7b from the bottom of the furnace vessel is in turn made considerably easier. The bottom electrode 7b - as can be seen from the dashed line - has a shaped body 15 made of a refractory material, which is arranged in a funnel-shaped recess within the bottom electrode 7b, the recess extending from the end face of the bottom electrode 7b facing the interior of the vessel in the axial direction extends to the area of the furnace vessel bottom 4. The shaped body 15 serves for the purpose of reducing the cross-section of the bottom electrode 7b in the direction of the interior of the furnace, the ratio of the cross-section of the bottom electrode 7a, b, c, d in the bottom 4 of the furnace to the cross-section in the furnace surface 16 being at least 1.4: 1. preferably 2: 1 was chosen. As a result, the liquid / solid boundary layer between the melt pool 13 and the end face the bottom electrode 7a, b, c, d in the furnace hearth surface is kept very stable and the service life of the bottom electrode 7a, b, c, d and the refractory lining 4 'of the furnace vessel bottom in the region close to the electrodes are significantly increased. If the cross section and the length of the bottom electrode 7a, b, c, d are adapted to the current flowing through them, with additional consideration of a factor, the power loss of the bottom electrode 7a, b, c, d can be reduced to a minimum.

Der mittlere Querschnitt ergibt sich dabei aus dem arithmetischen Mittelwert des Querschnittes der Bodenelektrode 7a, b, c, d im Bereich des Ofengefässbodens 4 und des Querschnittes in der Ofenherdfläche 16.The average cross section results from the arithmetic mean of the cross section of the bottom electrode 7a, b, c, d in the area of the furnace vessel bottom 4 and the cross section in the furnace hearth surface 16.

Im Ofengefässboden 4 weist die Elektrode 7b einen Durchmesser von d1 auf, der sich durch den Formkörper 15 in der Ofenherdfläche 16 bis zur radialen Ringbreite ds verringert. Die Elektrode 7b hat mitsamt den sie ergänzenden Teil 10 in der Ofenherdfläche 16 einen Durchmesser d7. Der mittlere Durchmesser ist mit d6 bezeichnet und die Länge wiederum mit I.In the furnace vessel bottom 4, the electrode 7b has a diameter of d 1 , which is reduced by the shaped body 15 in the furnace hearth surface 16 up to the radial ring width d s . The electrode 7b, together with the part 10 which complements it, has a diameter d 7 in the furnace hearth surface 16. The mean diameter is denoted by d 6 and the length is again denoted by I.

Fig. 4 zeigt die Bodenelektrode 7c, deren äusserer Durchmesser sich in Richtung Gefässinneres erweitert oder höchstens, durch die strichlierten Linien 7c' angedeutet, gleichbleibt. Die Bodenelektrode 7c weist kein sie ergänzendes Teil auf, weil die Auspressung nach dem Gefässinneren hin auch ohne dieses gewährleistet ist. Jedoch ist die Bodenelektrode 7c mit einem trichterförmigen Formkörper 15 versehen, welcher, wie bereits in der Beschreibung von Fig. 3 ausgeführt, der Stabilisierung der Grenzschicht Flüssig/Fest zwischen dem Schmelzbad 13 und der Stirnfläche der Bodenelektrode 7a, b, c, d sowie der Begrenzung der Verlustleistung dient. Die Bodenelektrode 7c hat im Ofengefässboden 4 einen Durchmesser d1. in der Ofenherdfläche 16 einen äusseren Durchmesser d8, jedoch in der Ofenherdfläche 16 nur eine metallisch wirksame Ringbreite dlo. Der mittlere Durchmesser ist mit dg bezeichnet, die Länge wiederum mit I.FIG. 4 shows the bottom electrode 7c, the outer diameter of which widens towards the interior of the vessel or at most remains the same, as indicated by the dashed lines 7c '. The bottom electrode 7c has no part that complements it, because the squeezing out to the inside of the vessel is ensured even without it. However, the bottom electrode 7c is provided with a funnel-shaped shaped body 15 which, as already explained in the description of FIG. 3, stabilizes the liquid / solid boundary layer between the molten bath 13 and the end face of the bottom electrode 7a, b, c, d and Power loss limitation. The bottom electrode 7c has a diameter d 1 in the bottom 4 of the furnace. in the oven surface 16 an outer diameter d 8 , but only a metallic ring width d lo in the oven surface 16. The mean diameter is designated with dg, the length with I.

In Fig. 5 ist eine Zweistoff-Bodenelektrode 7d dargestellt, welche aus einem der Ofenherdfläche 16 zugewandten Eisenlegierungsteil 31 und dem Kupferteil 32 besteht. Beide Teile 31, 32 sind metallurgisch miteinander verbunden, wobei die Auspressung der Elektrode 7b von Aussen in Richtung des Gefässinneren ohne weiteres gewährleistet ist.FIG. 5 shows a two-substance base electrode 7d, which consists of an iron alloy part 31 facing the furnace hearth surface 16 and the copper part 32. Both parts 31, 32 are metallurgically connected to one another, with the electrode 7b being pressed out from the outside in the direction of the interior of the vessel.

In Fig. 5 ist die Querschnittsverringerung der Bodenelektrode 7d in Richtung des Ofengefässinneren nicht dargestellt. Es versteht sich von selbst, dass ein entsprechender Formkörper 15, entsprechend Fig. 3 und 4 ebenfalls in die Bodenelektrode 7d eingesetzt werden kann.5, the reduction in cross section of the bottom electrode 7d in the direction of the interior of the furnace is not shown. It goes without saying that a corresponding shaped body 15, corresponding to FIGS. 3 and 4, can also be inserted into the bottom electrode 7d.

Im Schaubild gemäss Fig. 6 sind die Temperaturprofile der einzelnen geometrischen Ausbildungsformen der Bodenelektrode 7a, b, c, und zwar in qualitativer Darstellungsweise über deren Länge, veranschaulicht. Mit I ist dabei die Länge des metallischen Teils der Bodenelektrode bezeichnet, mit A die Temperatur der Bodenelektrode im Bereich des Bodens 4 des Ofengefässes und mit I die Temperatur in der Herdfläche 16.6 shows the temperature profiles of the individual geometrical forms of formation of the bottom electrode 7a, b, c, in a qualitative representation over their length. I denotes the length of the metallic part of the bottom electrode, A the temperature of the bottom electrode in the area of the bottom 4 of the furnace vessel and I the temperature in the hearth surface 16.

Die in Fig. 2 bis 4 dargestellten Ausführungsbeispiele der Bodenelektrode 7a, b, c weisen dabei folgende Temperaturprofile auf :

Figure imgb0001
The exemplary embodiments of the bottom electrode 7a, b, c shown in FIGS. 2 to 4 have the following temperature profiles:
Figure imgb0001

Die vorstehend aufgeführten Temperaturprofile zeigen folgendes :

  • Die zylindrische Ausbildungsform der Bodenelektrode 7b gemäss Fig. 3, jedoch ohne Formkörper 15 weist bei Stromfluss - Temperaturprofil E - über die Länge 30 im schmelzennahen Bereich der Bodenelektrode 7b einen sehr geringen Temperaturgradienten auf. Das bedeutet, dass die Grenzschicht Flüssig/Fest zwischen Schmelzbad 13 und Stirnfläche der Bodenelektrode 7b seine Lage während des Ofenbetriebes über den Abschnitt 30 in die Bodenelektrode 7b hinein verlagert, d. h., die Bodenelektrode 7b schmilzt ab und damit steigt auch die Verlustleistung an.
The temperature profiles listed above show the following:
  • The cylindrical design of the bottom electrode 7b according to FIG. 3, but without the shaped body 15, has a very low temperature gradient over the length 30 in the region of the bottom electrode 7b close to the melt when the current flows - temperature profile E. This means that the liquid / solid boundary layer between the molten bath 13 and the end face of the bottom electrode 7b shifts its position during operation of the furnace via the section 30 into the bottom electrode 7b, ie the bottom electrode 7b melts and the power loss also increases.

Ohne Stromfluss - Temperaturprofil C - steigt für dieselbe Bodenelektrode 7b (Zylindrische Ausbildungsform gemäss Fig. 3, jedoch ohne Formkörper 15) der Temperaturgradient über den Abschnitt 30 im ofenherdnahen Bereich wieder an und die Grenzschicht Flüssig/Fest zwischen Schmelzbad 13 und Stirnfläche der Bodenelektrode 7b bildet sich in Richtung Ofenherdfläche zurück.Without current flow - temperature profile C - for the same bottom electrode 7b (cylindrical design according to FIG. 3, but without shaped body 15), the temperature gradient rises again over the section 30 in the region near the oven and the liquid / solid boundary layer forms between the molten bath 13 and the end face of the bottom electrode 7b back towards the stove top.

Betrachtet man nun die Bodenelektrode 7a, gemäss Fig. 2 und Bodenelektroden 7b und 7c gemäss Fig. 3 und 4. deren Querschnitt sich in Richtung Ofenherdfläche 16, ausgehend vom Ofengefässboden 4 verjüngt, so ergibt sich folgendes Bild :If one now considers the bottom electrode 7a, according to FIG. 2 and bottom electrodes 7b and 7c according to FIGS. 3 and 4, whose cross-section tapers in the direction of the oven hearth surface 16, starting from the oven vessel bottom 4, the following picture results:

Mit StromflussWith current flow

Temperaturprofil D : Der Temperaturgradient in dem Abschnitt 30 im ofenherdnahen Bereich steigt gegenüber der Elektrode 7b mit zylindrischer Querschnittsform an, d. h. ein Abschmelzvorgang im Abschnitt 30 wird weitgehend zurückgedrängt. Die Temperatur der Bodenelektroden 7a, b, c - Temperaturprofil D - nähert sich dem Temperaturprofil F, d. h., der Temperatur des die Bodenelektrode 7a, b ergänzenden und hülsenartig sie umgebenden Teils 10, 10' aus feuerfestem Werkstoff, bzw. der feuerfesten Auskleidung 4' des Ofengefässbodens 4. Für einen elektrischen Ofen, der im industriellen Dauerbetrieb eingesetzt ist, ist dies von ausschlaggebender Bedeutung.Temperature profile D: The temperature gradient in the section 30 increases in the area near the oven compared to the electrode 7b with a cylindrical cross-sectional shape, ie a melting process in section 30 is largely suppressed. The temperature of the bottom electrodes 7a, b, c - temperature profile D - approaches the temperature profile F, that is to say the temperature of the part 10, 10 'made of refractory material or the refractory lining 4' which supplements and surrounds the bottom electrode 7a, b. of the furnace vessel bottom 4. This is of crucial importance for an electric furnace that is used in continuous industrial operation.

Ohne StromflussWithout current flow

Temperaturprofil B : Der Temperaturgradient im Abschnitt 30 verläuft sehr steil, die Grenzschicht Flüssig/Fest zwischen Schmelzbad 13 und Stirnfläche der Bodenelektroden 7a, b, c verbleibt stabil in der Ofenherdfläche 16.Temperature profile B: The temperature gradient in section 30 runs very steeply, the boundary layer liquid / solid between molten bath 13 and end face of bottom electrodes 7a, b, c remains stable in furnace hearth surface 16.

Generell ist festzuhalten, dass mit Bodenelektroden 7a, b, c, deren Querschnitt in der Ofenherdfläche 16 sich gegenüber dem Ofengefässboden 4 verjüngt, und zwar entsprechend dem erfindungsgemässen Dimensionierungsverhältnis, einmal der Temperaturgradient im herdflächennahen Abschnitt 30 der Bodenelektroden 7a, b, c erhöht werden kann. Damit wird erreicht, dass die Lage der Grenzschicht Flüssig/Fest zwischen Schmelzbad 13 und Stirnfläche der Bodenelektroden 7a, b, c weitgehend in der Ofenherdfläche 16 lokalisiert werden kann. Die Bodenelektrode 7a, b, c wird nicht abgeschmolzen und deren Lebensdauer wird wesentlich erhöht. Ausserdem wird durch diese Massnahme erreicht, dass die Bodenelektrode 7a, b, c besser auf minimale Verlustleistung dimensioniert werden kann.In general, it should be noted that with bottom electrodes 7a, b, c, the cross-section of which in the oven hearth surface 16 tapers in relation to the oven vessel bottom 4, specifically in accordance with the dimensioning ratio according to the invention, the temperature gradient in the section 30 of the bottom electrodes 7a, b, c near the hearth surface can be increased . It is thereby achieved that the position of the liquid / solid boundary layer between the melt pool 13 and the end face of the bottom electrodes 7a, b, c can largely be located in the furnace hearth area 16. The bottom electrode 7a, b, c is not melted off and its service life is significantly increased. In addition, this measure ensures that the bottom electrode 7a, b, c can be better dimensioned for minimal power loss.

Ausserdem liegen die Temperaturschwankungen der Bodenelektroden 7a, b, c, welche sich aus den Zuständen mit und ohne Stromfluss ergeben, näher an der Temperatur, der die Bodenelektroden 7a, b, c umgebenden feuerfesten Auskleidung des Ofengefässbodens 4.In addition, the temperature fluctuations of the bottom electrodes 7a, b, c, which result from the states with and without current flow, are closer to the temperature of the refractory lining of the furnace vessel bottom 4 surrounding the bottom electrodes 7a, b, c.

Dadurch werden thermische Spannungen in der feuerfesten Auskleidung weitgehend reduziert und somit der Gefahr deren vorzeitiger Zerstörung vorgebeugt.As a result, thermal stresses in the refractory lining are largely reduced, thus preventing the risk of their premature destruction.

Fig. 7 zeigt schliesslich das Temperaturprofil einer Zweistoff-Bodenelektrode gemäss Fig. 5, jedoch mit einem nach innen sich leicht verringernden Querschnitt. I gibt wiederum die Temperatur der Bodenelektrode 7d in der Ofenherdfläche 16 an, und A die Temperatur im gekühlten Abschnitt. Diese Verringerung des Querschnitts wird erreicht, durch Anbringung eines Formkörpers 15 innerhalb der Bodenelektrode 7d. Dieser Formkörper 15 wurde in Fig. 5 nicht dargestellt.FIG. 7 finally shows the temperature profile of a two-substance base electrode according to FIG. 5, but with a cross section that slightly decreases inwards. I again indicates the temperature of the bottom electrode 7d in the furnace hearth surface 16, and A the temperature in the cooled section. This reduction in cross-section is achieved by attaching a shaped body 15 within the bottom electrode 7d. This molded body 15 was not shown in FIG. 5.

G1 zeigt das Temperaturprofil des Teils aus Eisenlegierung, G2 dasjenige des Teils aus Kupfer. H gibt die Temperatur der feuerfesten Auskleidung 4' des Ofengefässbodens 4 wieder. Das Temperaturprofil G1 und G2 ist dem Temperaturprofil H angenähert. Hierbei zeigt sich die Wirkung der thermisch schlecht leitenden Eisenlegierung, trotz relativ grossem Querschnitts.G 1 shows the temperature profile of the part made of iron alloy, G 2 that of the part made of copper. H represents the temperature of the refractory lining 4 'of the furnace bottom 4. The temperature profile G 1 and G 2 approximates the temperature profile H. This shows the effect of the poorly thermally conductive iron alloy, despite the relatively large cross-section.

Claims (8)

1. Electric furnace, in particular a direct-current arc furnace for melting metals, with at least one bottom electrode (7a, b, c, d) which is cooled on its side facing away from the interior of the furnace vessel, characterized in that the ratio of the cross-section of the bottom electrode (7a, b, c, d) in the furnace vessel bottom (4) to the cross-section in the furnace hearth area (16) is selected to be greater than 1.4 : 1, preferably 2 : 1.
2. Electric furnace according to Claim 1, characterized in that at least one moulding (10, 10', 15) is provided which completes the bottom electrode (7a, b, c, d) to form a unit and which consists of a refractory material and has a conductivity lower than that of the bottom electrode (7a, b, c, d), that the shaping of the moulding (10, 10', 15) in at least a part region is adapted to the geometrical shape of the bottom electrode (7a, b, c, d), and that the moulding (10, 10', 15) has a cross-section which increases in the direction of the interior of the furnace vessel.
3. Electric furnace according to Claim 2, characterized in that the moulding (10, 10') surrounds the bottom electrode (7a, b) in the manner of a sleeve.
4. Electric furnace according to Claim 2, characterized in that the moulding (15) is located in a funnel-shaped recess within the bottom electrode (7a, b, c, d), the recess extending from the end face, facing the interior of the furnace vessel, of the bottom electrode (7a, b, c, d) in the axial direction approximately up to the region of the furnace vessel bottom (4).
5. Electric furnace according to at least one of Claims 1 to 4, characterized in that the bottom electrode (7a, b, c) is formed integrally and consists of an iron alloy.
6. Electric furnace according to one of Claims 1 to 4, characterized in that the bottom electrode (7d) is formed as a two-material electrode and comprises a metallic composite consisting, in the part facing the molten bath (13), of an alloy having chemical contents similar to those of the molten bath (13), and of copper in its other part, and that the part, facing the molten bath (13), of the bottom electrode (7d) corresponds to 1/8 to 1/2 of the total length of the bottom electrode (7d).
7. Electric furnace according to Claim 5 for iron alloy melts of 1 400 to 1 700 °C, and having liquid cooling, characterized in that the ratio of the mean cross-section to the length of the bottom electrode (7a, b, c) equal to f x I is selected such that 1/200 < f < 1/350, the mean cross-section being given in m2, the length being given in m, f being a temperature- and material-dependent constant in m/kA and I being the current in kA flowing through the bottom electrode (7a, b, c).
8. Electric furnace according to Claim 6 for iron alloy melts of 1 400 to 1 700 °C, and having liquid cooling, characterized in that the ratio of the mean cross-section to the length of the bottom electrode (7d) equal to f x I is selected such that 1/800 < f < 1/1 800, the mean cross-section being given in m2, the length being given in m, f being a temperature- and material-dependent constant in m/kA and I being the current in kA flowing through the bottom electrode (7d).
EP84108020A 1983-07-28 1984-07-09 Bottom electrode for a direct current arc furnace Expired EP0133926B1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AT84108020T ATE27059T1 (en) 1983-07-28 1984-07-09 BOTTOM ELECTRODE FOR A DIRECT CURRENT ARC FURNACE.

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
CH413483 1983-07-28
CH4134/83 1983-07-28

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EP0133926A1 EP0133926A1 (en) 1985-03-13
EP0133926B1 true EP0133926B1 (en) 1987-05-06

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ATE25179T1 (en) * 1983-07-28 1987-02-15 Bbc Brown Boveri & Cie PROTECTIVE ARRANGEMENT FOR A BOTTOM ELECTRODE OF A DIRECT CURRENT ARC FURNACE.
DE3461979D1 (en) * 1983-07-28 1987-02-12 Bbc Brown Boveri & Cie Cooling arrangement of a bottom electrode of a direct current arc furnace
DE3535692A1 (en) * 1985-10-05 1987-04-09 Gutehoffnungshuette Man CONTACT ELECTRODE ARRANGEMENT FOR DC ARC OR RESISTANCE MELTING OVENS
DE3535690A1 (en) * 1985-10-05 1987-04-09 Gutehoffnungshuette Man CONTACT ELECTRODE ARRANGEMENT FOR DC ARC OR RESISTANCE MELTING STOVE
CN103727799B (en) * 2013-12-14 2016-02-24 云南新立有色金属有限公司 A kind of using method of large-sized DC electric arc furnaces melting continuously high titanium slag electrode adding set
CN106091702A (en) * 2016-06-13 2016-11-09 巴涌 A kind of big furnace bottom power strength mineral hot furnace

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DE290945C (en) *
JPS5031524B1 (en) * 1969-12-25 1975-10-13
US3717713A (en) * 1971-02-18 1973-02-20 M Schlienger Arc furnace crucible
SE419929B (en) * 1974-11-25 1981-08-31 Asea Ab MELT CONTACT ELECTROD
FR2381987A1 (en) * 1977-02-23 1978-09-22 Asea Ab SOLE CONNECTION FOR A DIRECT CURRENT ARC OVEN
DE3106741C2 (en) * 1981-02-24 1983-06-16 M.A.N. Maschinenfabrik Augsburg-Nürnberg AG, 4200 Oberhausen Contact electrode arrangement for arc or resistance melting furnace

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DE3463568D1 (en) 1987-06-11
ATE27059T1 (en) 1987-05-15
EP0133926A1 (en) 1985-03-13

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